Terrain radar simulation



Dec- 10, 1963 E. E. GRAY ETAL TERRAIN RADAR SIMULATION 4 Sheets-Sheet 1 Filed July 8, 1960 INVENTORS EDWARD E. GRAY THOMAS P. PAPPAS RICHARD L. TAYLOR 7 AWK/ /mw Aornev WEDUEO Dec. 10, 1963 E. E. GRAY ETAL 3,113,989

TERRAIN RADAR SIMULATION Filed July 8, 1960 4 Sheets-Sheet 2 T6 /g EJLDIGITAL |:l` LOGIC 77 82 Ef- M 83 lOl 88 I 89 I 'T- TO 93 DISPLAY f CONSOLE 92 9| 93 O F lg. 2 +E OUTPUT VOLTAGE INVENTORS EDWARD E. GRAY THOMAS P. PAPPAS BY RICHARD L.TAYLOR Ma-yew E O INPUT VOLTAGE eAforney E. E. GRAY ETAL Dec. 10, 1963 TERRAIN RADAR SIMULATION 4 Sheets-Sheet 3 Filed July 8, 1960 EDW ARD E. GRAY MAS F? PA S O l HARD LTA AHornev Dec. l0, 1963 E. E. GRAY ETAL TERRAIN RADAR SIMULATION 4 Sheets-Sheet 4 Filed July 8, 1960 DDDD-UDDUDDDDDEDDDUD-UE UD-UD vnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnu RICHARD L. TAYLOR Mnl/d 0f.

Ammer United States Patent C) 3,ll3,939 ERRAFN RADAR SlMULA'ElN )Edward E. Gray, Mountain View, Thomas l. llappas,

Sunnyvale, and Richard: L. Taylor, San lose, @n/if.,

assignors to General Precision, lne., Binghamton, NX.,

a` corporation of Delaware Filed .lnly S, ltill, Ser. No. 41,5222 '7 Claims. (Ci. S25-11ml)v craft navigation, or the like, is well-known in the art and vbecause of its economy in time `and material, and the elimination of hazards of airborne teaching, such apparatus is currently widely used. Such apparatus may comprise generally, a unit which simulates or represents anv airplane and has a seat(s) for the student(s) positioned relative to an instrument panel with a complement ofinstr-unients simulating the aircraft performance instruvments of .a plane. Training apparatus may include simulationv or" any equipment, normally installed in an aircraft including auxiliary apparatus such as radar.

It is `an object or" this invention to provide an improved radar simulation wherein-a cathode ray tube may display terrain features such as hills `or mountains in a manner similar to an actual display by loperative radar equipment carried by an aircraft in flight, and wherein the terrain information is recorded on n photographic surface which may be scanned by a pick-up device.

A urther object of this invention is to provide a photov graphic'surtace wherein shades of gray may indicate terrain contours andelevations, and wherein a stand-ard gray scale is provided for digital calibration of specilic areas of the surface to correct for variations in the photographic processing 'of the surface; Another object is to provide a digital pick-up means forireceiving a digitally coded calibration signal corresponding to variations in the processing ofthe photographic film for correcting the video signals in accordance with the calibration signal thus picked-up.

Numerous other `obiectsand advantages will be a parent throughout the progress of the specification which yfollows. Theaccompanying drawings illustrate a certain selected embodiment of `the invention` 'and the views thereinl are as follows:

FIGURE 1 is a lschematic diagram of the optical systems and electrical circuits of the apparatus of this invention;

l FIGURE 2 isa further schematic diagram of the photo circuits and film cali ration circuits shown as blocks in FiGURiE 1; l

i FIGURE 3 illustrates sample lilin correction curves obtainable from the apparatus of EIC-URE 2;

Pce

FIGURE 4 is a diagram of film transport apparatus and the optical systems of this invention;

FIGURE 5 illustrates an enlarged portion of the photographic surface or hlm used in this apparatus.

Brie-'ily stated, according to this invention, a photo graphic surf-ace or film strip 'll is scanned by means such as la flying spjot scanner l@ having a cathode rayV tube 13 for generating a spot of light which is focused upon the lin strip ll by optical means d4. The lm strip includes a map section wherein various shades of gray represent contour elevations, `and a photosensi-tive pick-up means such as a photo-multiplier lo develops a video signal v from Ithe scanning of the tfilrn stnip lll. The video signal. is passed by an amplifier 17 and other circuits 'ultimatelyy to a display console le including means such as va cathode ray nube for reproducing la simulated radar display. Means are provided for calibrating or correcting the video signal to compensate for variation in nlm density vor grayness pattern resulting from photographic processing thereof. K

The flying spot scanner l2 develops -a spot of light on the face plate of ythe cathode ray tub-e d3 aV conventional manner by an electron beam impinging uponv a phosphor layer. Because phosphore are granular, the layer is somewhat non-uniform, and therefore, the light emission is likewise non-uniform as the electron beam scans across the layer. The means provided for correcting or Calibrating the video signal to compensate for the non-uniform light emission includes another photo-friultiplier 2li optically arranged for directly sampling the light output of the ily-ing spot scanner i2. one optical system associated with the photo-multiplier Ztl may include :a beam splitter or dichroic mirror 2l, a lens 22 and a ine-ans for attennating the light sucn as a clear lilm panel 213. The output from the photo-multiplier illyl is passed by an amplifier 2d to an analog ividing circuit 25. The dividing circuit 25 receives the video signal from the elevation read-out channel including the ampliiier l `together 'with the signal from the photo-multiplier monitor Ztl', and performs an analog division such that lthe elevation video signal is attenuated by an amount corresponding to the instantaneous brightness of `the ilying spot scanner. The signal passed from the analog dividing circuit 25 corresponds to lan elevation video signal which may have been developed directly had the ilying spot scanner l2 produced a constant light output,

and therefore, the circuitry including the photo-multiplier Z@ and the divider 25 constitutes a means for `cornpearsating for variations i-n the illumination from the scanningmeans l2.

The photo-multipliers loy and Ztlk are inherently'subject to drift variations such that the electrical signal developed .for fa given light input tends to vary `slowly or drift. A synchronized means for periodically correcting drift variation and re-calibnating the photo-multipliers" is provided by'a standard source of illumination 27 op@` tically coupled to both the photo-multipliers lo and 2li.

The illumination source may be la tube placed withinLa cylinder which is mechanically coupled to and rotated by a synchronous motor 129. The cylinder Z8 isgenerally opaque but has ia longitudinal extending transparent slit or opening 3%* therein. it may be appreciatedV that a flat beam or plane of light will be emitted and will sweep about the cylinder Z3 as it rotates. A pair of light pipes or Lucite rods 32 and 33 connec-L with appropriate ports in an outer housing surrounding the rotating cylinder 218 and constitute the means for passing light from the standard light source to the photo-multipliers lo and 20. Obviously, the light passed to the photo-multipliers is synchronized with the rotation of the cylinder 2.3 and will appear upon the photo-multiplier circuits during retrace intervals when the flying spot scanner l2 and the display console |18 are blanked out and rendered inoperative.

The housing 3e contains a pair of ports for passing light to the photo-multipliers i6 and Ztl and contains a port and further optical arrangement for passing light to a third photo-multiplier and timing circuit 3S. The photo-multiplier circuit 55 receives pulses of light synchronized by the cylinder 26 and functons -to generate electrical timing pulses for the remainder of the circuitry. Timing pulses from the circuit 35 are passed to doffe-:tion circuits 35 wherein deflection signals are generated for both the flying spot scanner l2 and the display console i8. lFurther timing pulses are passed to AND gates Se and 40 whereby the gates are conditioned to pass signals during particular times with respect to the rotation of the cylinder 2S and the deflection signals. Therefore, the rotating cylinder 2S constitutes a master timing means for generating timed illumination pulses both for calibration of the photo-multipliers 16 and 29 and for generation of electrical timing pulses.

During re-trace intervals the standard illumination from the source 27 causes the photo-multipliers 16 and 2lb to produce an output volta ge for calibration purposes-which output voltage is passed by the respective amplifiers i7 and 25.". During the re-trace intervals, the AND circuit or gate 36 is conditioned to pass the signal from the amplifier 17 to a feedback circuit including a memory device 37 and a direct current amplifier 38 to control a dynode voltage of the photo-multiplier i6. Since the voltage output from a photo-multiplier depends largely upon the acceleration voltages applied to dynodes or secondary emissive elements, a gain control may be provided for the photo-'multiplier by control of the dynode voltages. Therefore, the signal level passed from the photo-multiplier lo during re-trace intervals is fed back through a memory device 37 and an operational amplifier 38 to control the gain of the multiplier and to effectively establish the output voltage therefrom at a pre-set value. The memory device 37 may be a simple resistancecapacitance integrating circuit `for storing a voltage from one re-trace interval to the next. During a re-trace interval,

when the gate 36 is conditioned to conduct, the feedback circuits 36, 37 and 38 become operative to establish a desired output voltage from the photo-multiplier 16 whereupon an appropriate feedback voltage is stored in the memory circuit 37. Between retrace intervals the gate 36 is rendered non-conductive, and the feedback voltage to control the dynodes of the photo-multiplier is merely retained in the memory 37. Thus, the photomultiplier is calibrated duringthe re-trace times when both the flying spot scanner l2 and the display console 2 8 are blanked out and rendered inoperative.

The photo-multiplier 2t) is likewise subject to drift variation, and therefore, a similar feedback circuit is provided to stabilize the output voltage therefrom, This feedback circuit includes the gate dll, a memory circuit 41 and an amplifier 42. ln both feedback circuits the amplifiers 35 and 42 respectively include a manual gain adjustment 43 and ifi for establishing a desired output level.

As indicated heretofore, elevation contour information is processed on a filmstrip wherein various shades of gray represent various elevations. Since this apparatus simulates radar display as an airplane moves, means are provided for moving the film ll with respect to the flying spot scanner. The forward motion of the aircraft may be considered as an X direction and such movement may be accomplished by passing the film ll over a drive roll 46 under the control of a servo drive generally shown as a block fr? (PEG. l). rthus, the velocity of the drive roll is controlled by an X servo mechanism from an X input and may be derived from simulating flight control of the training apparatus (not shown). Although the simulated flight path is generally longitudinal or in the X direction with respect to the lm, such flight path may deviate from the ideal center line of the film whereupon a Y drive component may be introduced. The Y drive or transverse movement is accomplished by physically moving the flying spot scanner l2 and its associated optical system lll transversely with respect to the film` ll.

As shown in FlGURE 4, the film 1l is trained about a pair of sprocket drive rolls 46 and i3 together with idler rolls 49 and Sli on each side of the photo-multiplier A first drive motor X accomplishes movement of the film through a pair of gears 52 and 53 which drivingly couple the motor X to the sprocket roll 46. Reversal of the hlm may be accomplished since the motor X is also couplied to the rrive sprocket 4S via the gears S2, 54, 55 and A safety clutch S7 may be included in the drive arrangement. A pair of reels 59 and 60 will feed film to and take up film from the respective drive sprockets 46 and 4S.

A linear motor Y is directly connected to move the flying spot scanner carriage l2 along a pair of ways 62 and 63. The beam splitting dichroic mirror 2l and the lens 22 together with another mirror oel are supported by a bracket 65 fixed to the flying spot scanner carriage l2.

FGURE 5 shows a section of the film il, and it is to be noted that a principal area 67 represents a map with contour lines appearing thereon. The area 63 constitutes a gray scale band and contains standard contours extending longitudinally of the film.

The map portion of the film may be prepared by a successive stripping technique. This technique is fully described in a bulletin entitled Dystrip Technique of Color Separation by the Aeronautical Chart and Information Center, Air Photographic and Charting Service (MATS). Generally this technique involves photographing contour lines from a map onto la negative, and then hand stripping the emulsion from the film in particular areas representative of elevations. The stripping process is alternated with an exposure of the final film strip through the windows or specific openings of the stripped away material from the negative and then to darken a positive film overlying the negative. After each successive exposure of the diapositive transparency, a further contour elevation area may `be stripped from the negative. The areas of highest elevation will be first stripped from the negative and first exposed upon the positive film. Successive exposures of the positive will include successively lower elevations together with the higher elevations which have been previously exposed. Therefore, the diapositive film map area will be longer exposed and darker in those regions of high elevations while the regions of low elevations will be lighter in character. The mountain tops will be black and the low valleys will be white with various shades of gray representing elevations therebetween.

Since photographic film processing techniques are subject to density vairations, the radar simulation apparatus of this invention provides a means for electronic compensation of the signals picked up from the film strip.

.e gray scale band area 68 (PEG. 5) is photographically processed together with and identical to, the map area 67. The darker portion of the gray scale band on the left side thereof corresponds with the highest elevations or mountain peaks of the map area and the contour gradation extends across the gray scale hand such that the right side thereof corresponds to the lowest elevations of the map. lt would be possible to determine the elevation of any point on the map by a comparison with the grayness value of that point to the gray scale representing standard elevations, since the photographic processing Was the same vin both cases and any grayness error resulting Yfrom the templates a digital calibration of the film.

An area or band 7d extending along the right side of the film strip (as shown lin FIGURE 5) includes four tracks :for digital indicia which may appear as opaque dots 71.r The indicia 71 constitutes a four-bit coded rep resentation corresponding to a grayness-,characteristic of Ithe tilm as it may be scanned from one side of the gray scale dS to the other. This `grayness chanacteritsic is generally non-linear and resul-ts from many Variables of photography and associated with the film processing.

' Further incorporated in this characteristic may be the pick-upresponse characteristic of the photo-multiplier ld, and the output response characteristic of the display con- Y sole 15. Thus, the Vdigital value represented by the fourbit code 7l is initially obtained by examination of the gray scale band, is representative of the non-linear characteristics of the photographic film and further includes characteristics of the photo-multipliers and display console.

The gray scale area 63 is used to establish the digital co-ding bits 71 corresponding to variation in film density, and once the coding 7d is established, the ygray scale band 63 is of no further value. lt has been suggested that the gray scale be positioned on each side `of the lin at the extreme edges thereof. After the fiimhas been developed and the digital coding 7l established, a final operation is to cut the sprocket holes along the edge. Ihus, the edge portion 72 may be first used as a gray scale and then sube and the eiective feedback resistance of the amplifier 86 is controlled by the relays 33` and d4 which operate in accordance with the digital information received from the film strip. The digital logic circuits S2 will include ampliliers for each of the channels and may inoludea so# phisticated logical arrangement of AND and YOR gates, but in a simpler form, the amplified signals `from. the photo-sensitive device 77 may be passed directly to corlresponding relay windings and 84. Thus,y in this simple form, the indicia `7J; (FEGURE 5) will directly control which o the respective ones of relays 8f3 and 8d will close and which others will remain open. The relays may be of a latching type, whereupon short duration Signals from the photo-cells 77 Will result in a holding of certain switches and resistors in the amplifier circuit.

As indicated heretofore, the function of the 4amplifier do and its associated logic and relay circuits is to modify the video signal in accordance with variations arising from the nlm processing. The manner in which the video signal must be modified 'for a specific por-tion of the film must be determined by a study of the film characteristics for that film portion to determine complementary characteristi-c curves. FIGURE 3 illustrates two possible cor reotion curves wherein the video signal input is plotted against the desired video signal output for twoassurned conditions. Obviously, further curves may be developed for :other desired characteristics. Bach of the curves of FlGURE 3 illustrate the amplifier output having four nearly linear segments of different slopes, and by a proper choice or" input resistance and `feedback resistance, the gain of the amplifiery S6. will determine the slope of each line segment. rlhus, the choice of the resistors 87 through sequently used for sprocket holes to effect an economy in n Y yfilm area. and/or .to permit the map area 6'7 to be *enlarged In FlGURE 1 the digital tracks 7l) are illuminated by a narrow beam of light which is generated by a source. 73 and shaped by a slit aperture 74 and then focused upon the track "i0 by optical means 75. Further optical means ,76 may focus incremental areas from the four digital tracks 70` of the film strip 1l upon four photo-sensitive devices 77. The photo-sensitive devices 77 may be of conventional nature, and it has -been contemplated to use photo transistors in this circuit. The photo cells or photo transistors f7 will produce digital electrical signals'which may `be a-mplied by photo circuits 78 and passed to a.

film calibration circuit 79, FIGURE 1. The film calibravtion cincuit 79 may include an amplifying means for passing the video signal from the divider 2d to lthe display .console i3 `and to a clearance plane computer and radio altimeter tid.

The clearance plane computer and radio .eter circuits are more yfully disclosed in the Patent No. A3,081,774 Serial No. 41,564, Supra entitled Radar Simulation issued to Mr. Edward E. Gray, Mr. Keith'l. McFarland and Mr. Kenneth il. Hackett.

.FIGURE 2 illustrates more specifically 'the photo logic circuits 7S and the calibration amplifier 79. The photo cells Aor photo .transistors 77 may' pick-up specific digital information from the four digital tracks 7@ (FIGURE 5),

' and the output from the photo cells is passed to digital logic circuits S2. The digital logic circuits 82 may include ampliiiers yfor ythe four digital signals which will selectively pass output currents through respective relayv windings S3 and 3d. The video Isignal input from the divider 25 will appear at an input terminal d5' which is coupled to an operational amplifier 86 fby at least one resistor @f7 and by further resistors 3S, S9, gli, 9d and 92- which will constitute selective parallel paths .for current flo-w providing respective relay contacts 93 are closed. The operational amplifier 8d will have a high forward or openV loop gain and will include a yfeedback resistor 95 and further possible feedback resistors 96, 97, 98, 99, ldd-and itil under control of associated relay contacts 02, lltl, ldd and 165 together withy a pair of diodes 106 `and M7. Obviously, both the effective input resistance 9d. and 9S through lill will permit a designer to lforni a desired curve (FIGURE 3) closely approximating an ideal video correction curve. i

The curve to the left of the E0 input voltage (FlGURE 3) corresponds to relatively low values of input voltage` applied to terminal (FIGURE 2) and when the input is low, the 'diodes 9d, 15516 and 1&7 will remain cut ofi whereupon certain of the resistors will not be coupled into the circuit. As the scanning beam sweeps from a loW value Iacross, the film strip to a darker value of grayness, the input voltage at the terminal S5' will increase, and at a point E0 the diode i -Will conduct to couple further resistors into the input circuit of the .amplifier 86. As the beam continues to sweep and the video signal at the point 35 continues to increase, the output voltage ata point le@ will increase to a value El and the diode 107 may conduct. Diodes lt and 167 may be biased nega-Y tively from the output terminal 09' by a sourceV negative voltage E applied through a potential dividing resistive network including resistors lll, Ml and H2 las the signal level continues to increase the diode .lti'd may conduct'.

whereupon further resistors :are connected into the feed,- baclr circuit of the amplier 86.

rae output video sigaar at terminar un' win follow a' function curve Isimilar in form to the curves of FIGURE 3 such that as the' voltage increases or decrease-s during a single scan of the dying spot scanner l2' the diodes 94, 166 and 25.67 may cut in and out depending upon the signal level. As the flying spot scanner progresses along the film strip il in the X or longitudinal direction, different respon-se characteristics may be demanded i because of variation in the grayness characteristics of the filmrwhereupon the photo cells 77 will respond to the indicia 7l Z or less than is shown in FiGUl-E 2 depending upon the processirigl variations of the film strip being used.

From lthe foregoing it may be appreciated that the radar display simulation apparatus continually corrects for variation in the film density or grayness, for variation in the brightness of the flying spot scanner l2, and for variation or drift in the photo-multiplier circuits. Since the drift variation of the photo-multipliers is comparatively sl iv as compared with the scanning rate, the feedback circuit including the memory 3? and the amplier 33 may pass only low frequencies and direct currents. However, the analog dividing circuit 25 must be extremely fast in operation since the light intensity of the flying spot scanner l2 may vary at a 500 kilocycle rate. This circuit is contemplated to be similar to a logarithm multiplier shown and described on page 59 of Analog Nethods Computation and Simulation hy W. I. Karplus and W. W. Soroka published by McGraw-lill Bool: Company, l95,. This circuit converts both inputs into logarithmic onm, performs an analog subtraction, and then converts the output to the antilogarithm. Fast operation of this circuit is obtained by using only a single stage amplifier with a large feedback cuir-rent. Since the amplification is accomplished in a single stage, the resulting phase shift and roll-off characteristics of the amplifier are minimized, and the response time is greatly reduced. The amplifier do, for providing a film density, or H and D conrection must lilrewise be of fast operation for passing signals and for providing diode step corrections in the video signal,

Changes may `be made in the form, construction and arrangement of Ithe parts Without departing from the spirit Iof the invention or sacrificing any of its advantages, and the right is hereby reserved to maire `all such changes as fall fairly within the scope of the following claims.

The invention is claimed `as follows:

l. Apparatus for simulating a radar display of terrain, said apparatus comprising a photographically processed surface whereon contours are represented as shades of gray, a means for scanning the surface, a pick-up means sensitive to the scanning of the surface for generating an electrical signal corresponding to the shade of gray being scanned, said surface having digital calibration indicia corresponding to grayness variation or the photographic process of the surface, and amplifying means electrically coupled to the pick-up means and controlled by the digital calibration indicia whereby the electrical signal is calibrated to represent terrain contours.

2. Apparatus for simulating a radar display of terrain, said apparatus comprising a photo-graphically processed .lm whereon contours are represented as shades of gray, a means for scanning the film with a beam of light, a photo-sensitive means positioned to receive the scanning beam and operable to generate an electrical signal corresponding to the shade of gray being scanned, said hlm being calibrated by pliotographically reproducing a standard gray scale band and thence providing a digital indication corresponding to the photographic reproduction of the band, amplifying means electricallv couple-:l to the photo-sensitive means, and a digital pick-up means responsive to the digital indications and contrcllably coupled to the amplifying means whereby the electrical signal is amplified in accordance with the photographic calibration :of the film.

3. ln apparatus for simulating a radar display of terrain, a photographically prepared surface comprising a map arca including portions of varying shades of gray representative of map contours, a gray scale area including portions of reproduced varying shades of gray corrosponding with standard contours, and digital indicia positioned in spaced relation with incremental portions of the map area and with incremental portions of the gray scale area, the arrangement of said digital indicia providing coded information corresponding to that of said stand- .ard coni-ours, adapted for calibration of said map area.

S ratus for simulating a radar display of terrain, eoraratus comprising a surft.: having a map arca shades of gray are photographically processed represent terrain contours, a flying spot scanning means 'tor generating a light beam to scan the surface, a photon receiving scanning beam from the tive means urface for developing an output signal in accordance with the shades of gray, a first calibration means for correctg the output signal in accordance with variations in the prrotographic processing of the surface, and a second calibration means for correcting the output signal in accordance with variations in the light output from the flying spot scanning means, said first calibration means including a digital representation on the surface derived from a standard gray scale photographically processed with the map area and a means for sensing the digital rercsentation, said second calibration means including another photo-sensitive device optically positioned to receive light from the flying spot scanning means.

5. Apparatus for simulating n radar display of terrain, sait. apparatus comprising a surface havin,f7 a map arca rein shades of gray are photographically processed to cscnt terrain contours, a flying spot scanning means for generating a light beam to scan the surface, a photomult: lier for receiving the scanning beam from the surface for developing an output signal in accordance with the shades of gray, an amplifier circuit coupled to the photo-multiplier for passing the output signal, a first calibration means for correcting the output signal in accordance with variations in the photographic processing of the surface, a second calibration means for correcting the output signal in accordance with variations in the light output from the flying spot scanning means, and a third calibration means controllably coupled to the photo-multiplier for correcting the output signal in accordance with drift variations of the photo-multiplier, said first calibration means including a digital representation on the surface derived from a standard gray scale photographically processed together with the map area and a means for sensing the digital representation controllably coupled to the amplifying means, said second calibration means iucluding another photo-multiplier optically positioned to receive light from the flying spot scanning means and controllably coupled to the amplifying means, said third calibration means including a standard light source and optical means for periodically applying the standard light source to the photo multipliers for correcting drift variations therein.

6. Apparatus for simulating a radar display of terrain, said apparatus comprising a photographically processed film whereon contours are represented as shades of gray, a means for scanning the film with a beam of light, a photo-sensitive means positioned to receive the scanning beam and operable to generate an electrical signal corresponding to the shade of gray being scanned, said film being calibrated by photographically reproducing a standard gray scale band and thence providing a digital indication corresponding to the photographic reproduction of the band, amplifying means electrically coupled to the the photo-sensitive means, said amplifying means including a plurality of impedance elements for passing input signals and a further plurality of impedance elements for passing feedback signals, and a digital pick-up means responsive to the digital indications and controllably coupled to the amplifying means, said amplifying means further including switching means controllably coupled to the impedance devices and responsively coupled to the pick-up means whereby the digital indications cause operation of the switching means and selectively establish an input and a feedback circuit to provide desired characteristics in the amplider in accordance with the photographic calibration of the film.

7. Apparatus for simulating a radar display of terrain, said apparatus comprising a photographically processed `lrn whereon contours are represented as shades of gray,

a means for scanning the lm with a beam of light, a photo-sensitive means positioned to receive the scanning beam and operable to generate an electrical signal corresponding to the shade of gray being scanned, said lm being calibrated by photographically reproducing a standand gray scale band and thence providing a digital indication corresponding to the photographic reproduction of the band, amplifying means electrically coupled to the photo-sensitive means, said amplifying means including a plurality of resistance elements for passing input signals and a further plurality of resistance elements for passing feedback signals, and a digital pick-up means responsive to the digital indications and controllably coupled to the amplifying means, said amplifying means further including selective switching means controllably coupled to the 15 resistance elements and responsively coupled to the digital pickup means, said amplifying means further including at least one unidirectional conduction device coupled to one of the resistance elements and operable to connect the resistance element into the amplifying circuit only when the signal attains a pre-determined level whereby the electrical signal is ampliiied in accordance with the photographic calibration of the lm.

References Cited in the le of this patent UNITED STATES PATENTS Weddington Jan. 28, 1941 2,563,892 Waller Aug. 14, 1951 2,603,912 Gruber July 22, 1952 2,720,039 Brown Oct. l1, 1955 2,737,730 Spencer Mar. 13, 1956 2,783,454 North Feb. 26, 1957 2,788,588 Lindley Apr. 16, 1957 2,870,548 Chedister Ian. 27, 1957 

1. APPARATUS FOR SIMULATING A RADAR DISPLAY OF TERRAIN, SAID APPARATUS COMPRISING A PHOTOGRAPHICALLY PROCESSED SURFACE WHEREON CONTOURS ARE REPRESENTED AS SHADES OF GRAY, A MEANS FOR SCANNING THE SURFACE, A PICK-UP MEANS SENSITIVE TO THE SCANNING OF THE SURFACE FOR GENERATING AN ELECTRICAL SIGNAL CORRESPONDING TO THE SHADE OF GRAY BEING SCANNED, SAID SURFACE HAVING DIGITAL CALIBRATION INDICIA CORRESPONDING TO GRAYNESS VARIATION OF THE PHOTOGRAPHIC PROCESS OF THE SURFACE, AND AMPLIFYING MEANS ELECTRICALLY COUPLED TO THE PICK-UP MEANS AND CONTROLLED BY THE DIGITAL CALIBRATION INDICIA WHEREBY THE ELECTRICAL SIGNAL IS CALIBRATED TO REPRESENT TERRAIN CONTOURS. 